There exist two commonly implemented front-end architectures in radio frequency (RF) receiver design; namely, the homodyne architecture and the superheterodyne architecture. The homodyne architecture down-converts a desired carrier (or channel) in a received signal directly from RF to baseband, whereas the superheterodyne architecture down-converts a desired carrier (or channel) in a received signal to one or more intermediate frequencies (IFs) before down-conversion to baseband. In general, each of these front-end architectures typically employ an antenna to receive a signal, a low noise amplifier (LNA) to provide gain to the signal, and one or more down-conversion and filtering stages.
In both front-end architectures, the down-conversion stage(s) include a mixer for mixing the received signal with a local oscillator (LO) signal to down-convert the desired carrier in the received signal to baseband or some non-zero IF for further processing. The LO signal in the homodyne architecture is ideally tuned to have a frequency identical to the desired carrier such that the carrier is down-converted to baseband. The LO signal in the superheterodyne architecture (or at least one LO signal in the superheterodyne architecture), on the other hand, is ideally tuned to have a frequency that is offset from the frequency of the desired carrier by an amount equal to the chosen IF such that the carrier is down-converted to the IF.
There is often a small frequency error (or offset), however, in the LO signal from its ideal frequency. This error causes the desired carrier to be down-converted to a frequency position other than what is expected (i.e., to a frequency position other than at baseband in a homodyne architecture and to a frequency position other than at the chosen IF in the superheterodyne architecture). Proper recovery of the information modulated onto the desired carrier generally requires that the carrier be down-converted (very close) to the expected frequency location.
Therefore, automatic frequency correction is often employed at the receiver to estimate and compensate for any frequency error in the LO signal, such that the desired carrier is down-converted (very close) to its expected frequency position (i.e., very close to baseband in a homodyne architecture and very close to the chosen IF in the superheterodyne architecture). FIG. 1 illustrates a conventional homodyne receiver 100 that performs automatic frequency correction. As illustrated in FIG. 1, conventional homodyne receiver 100 includes a front-end 105 for performing amplification, down-conversion, and filtering, and a baseband processing section 110 for performing decoding or demapping.
Front-end 105 specifically includes an antenna 115, a low-noise amplifier (LNA) 120, a mixer 125, a phase-locked loop (PLL) 130, a digitally controlled crystal oscillator (DCXO) 135, a crystal resonator 140, a low-pass filter 145, an analog-to-digital converter (ADC) 150, and a digital signal processor (DSP) 155. In operation, antenna 115 is configured to receive an RF signal that includes a desired carrier. The desired carrier can be positioned within a frequency band defined by a particular communications standard. For example, the desired carrier can be positioned within a frequency band defined by the Global System for Mobile Communications (GSM) standard.
After being received, the RF signal is provided to LNA 120, which provides sufficient amplification to the RF signal to overcome the noise of subsequent stages in front-end 105, for example. The amplified RF signal is then mixed by mixer 125 with a LO signal provided by PLL 130. PLL 130 provides the LO signal based on a reference oscillator signal provided by DCXO 135 and crystal resonator 140 (i.e., PLL 130 provides the LO signal as some multiple or fractional multiple of the reference oscillator signal). The LO signal is ideally controlled by PLL 130 to have a frequency equal to the desired carrier such that the mixing operation, performed by mixer 125, results in the carrier being down-converted to baseband. The down-converted signal is then filtered by low-pass filter 145 to remove unwanted frequency components, converted to a digital signal (i.e., a sequence of discrete values) by ADC 150, and processed by DSP 155.
Baseband processing section 110 receives the down-converted and filtered signal from DSP 155 and performs further processing. As illustrated in FIG. 1, baseband processing section 110 includes baseband processor 160 and an automatic frequency controller (AFC) 165. Baseband processor 160 is configured to perform decoding or demapping to recover information transmitted over the carrier. AFC 165 is configured to estimate and compensate for any frequency error in the LO signal provided by PLL 130, such that the desired carrier is down-converted (very close) to its expected frequency position (i.e., very close to baseband in the homodyne architecture illustrated in FIG. 1).
AFC 165 can estimate the frequency error using, for example, the down-converted carrier or the information recovered from the carrier by baseband processor 160. The estimated frequency error can then be used by AFC 165 to adjust a frequency at which crystal resonator 140 oscillates and, thereby, the frequency of the reference oscillator signal. Specifically, AFC 165 can adjust the frequency at which crystal resonator 140 oscillates using DCXO 135. For example, DCXO 135 can include a tunable capacitor coupled in parallel (or series) with crystal resonator 140. In general, adding and removing capacitance across a crystal resonator, such as crystal resonator 140, will respectively cause the resonance of the crystal resonator to shift upward and downward.
As noted above, the reference oscillator signal provided by DCXO 135 and crystal resonator 140 is used by PLL 130 as a reference signal to generate the LO signal used by mixer 125. Thus, the frequency of the reference oscillator signal provided by DCXO 135 and crystal resonator 140 can be adjusted by AFC 165 to compensate for the estimated frequency error in the LO signal provided by PLL 130.
Although adjusting the frequency of the reference oscillator signal presents a viable solution for reducing the frequency error in the LO signal provided by PLL 130, this solution has drawbacks. One notable drawback is that the reference oscillator signal provided by DCXO 135 and crystal resonator 140 potentially can no longer serve as a reference clock for other functionalities supported by the device containing homodyne receiver 100. For example, many communication devices, such as cellular phones, provide support for wireless local area network (WLAN) and Global Positioning System (GPS) functionalities in addition to cellular communication functionalities. Even though the reference oscillator signal of a single crystal resonator, such as crystal resonator 140, can be used to support each of these additional functionalities, the sudden changes in frequency of the reference oscillator signal that are caused by AFC 165 to support one functionality are not acceptable to many of the other functionalities, such as GPS.
Therefore, what is needed is a system and method for performing automatic frequency correction in a receiver without adjusting the resonance of the crystal resonator.
The present invention will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number.